1. Field of the Invention
The present invention relates to an office machine with an arrangement for adjusting the spacing of a printing head from a platen as a function of the number of copies of the original that are being made, the head being a wire-type head.
2. Description of the Prior Art
Devices for adjusting the distance of the printing head from the platen are known; in particular, an adjusting device is mounted on a carriage and can be shifted in a direction perpendicular to the axis of the platen. In the rear part of the carriage (the part facing the platen), there is pivoted a pulley which rolls on a flat auxiliary guide fixed to the frame of the machine. On the auxiliary guide there are placed strips of a thickness equal to the total thickness of the sheets placed around the platen, so that when the pulley rolls thereon the head is moved away from the platen by an amount exactly equal to the thickness of the sheets on which it is desired to print. As can be seen, such an adjustment of the distance of the head from the platen is very complicated and requires a long time for the setting up.
There is known a printer wherein the printing heat is slidable on guides movable perpendicularly with respect to the platen and wherein the transporting element for the head is constituted by a belt driven by a motor and connected directly to the head. When the guides are shifted, the belt bends transversely to accommodate the perpendicular adjustments of the head.
This arrangement, however, has the disadvantage that owing to the perpendicular movements of the head, the driving belt is stressed by considerable forces at the beginning and at the end of printing operations, and the belt is therefore liable to wear out very quickly. Moreover, as regards construction, the free length of the belt is necessarily greater than the length of the printing line.